1. Field Of The Invention
This invention relates generally to varistors, and more particularly to multilayer ceramic varistors having sputtered terminations.
2. The Related Art
"Varistors" or voltage-dependent nonlinear resistors have been used as, among other things, surge absorbing elements, arresters and voltage stabilizer elements. Varistors typically employ single layer, disk-shaped ceramic bodies having voltage-dependent nonlinearity. Multilayer varistors became available in the market in 1988. The preparation and typical composition of this type of varistor is described in detail in U.S. Pat. No. 4,290,041 to Utsumi et all. which is incorporated herein by reference. The varistor may comprise a semiconducting block-shaped body made up of conducting grains separated by voltage sensitive grain boundaries. The ceramic material is typically a zinc oxide blend. After formation of the varistor's ceramic body it is necessary to "terminate" the varistor, that is, to apply conductive coatings to the exposed electrode portions of the varistor. This permits the varistor to be readily connected to a printed circuit board or the like.
In a typical method of manufacturing varistors, termination is achieved by applying paste to the surfaces of the ceramic body having exposed electrodes. The paste may comprise a low melt glass frit and a conductive material such as silver or a silver alloy. After application of the paste, the varistor is heated to drive off solvents and/or binders and to fuse the glass and silver composition to the ceramic body. The terminated varistors may have electrical leads soldered to them or be used as surface mount devices.
The described method has drawbacks. First, the terminating compound can be very costly if palladium, platinum, and/or other noble metals are added to improve leach resistance. The cost of the added materials can be ten times or more than that of silver alone. Improving leach resistance is necessary as nickel-plated terminations have become an industrial standard for surface-mount components. Second, even when palladium and other noble metals are added to the termination compound, the resulting leach resistance will not be as good as that of the terminations including a nickel barrier. To reduce cost and improve leach resistance, attempts have been made to provide varistors with a nickel barrier by a plating operation. For example, a plurality of varistors already terminated and fired with silver terminations may be placed in a plating basket in a known technique for plating ceramic capacitors. The basket is then immersed in a plating solution. After sufficient metal has been deposited, the basket is removed from the plating solution and the varistors are cleaned.
This plating operation has a number of drawbacks. One difficulty resides in the fact that the varistor is a semiconductor made up of conducting grains separated by voltage sensitive grain boundaries. This sensitivity to voltage change subjects the varistor to "creepage" during the plating process. Creepage is the phenomenon where plating covers not only the end portions of the body (as it is supposed to), but begins to plate or "creep" from the end portions across the entire body from end to end. Of course, when the creepage reaches from end to end shorting occurs and the varistor is useless. This problem can be eliminated by applying an insulating compound such as a plastic binder over the areas where plating is not desired. However, this requires an added step to the process and adds to the manufacturing costs. Furthermore, the plating solution is generally acidic and will gradually etch the ceramic body if contact is made during plating.
Additionally, a zinc oxide varistor may degrade when subjected to elevated temperatures in a reducing atmosphere during processing, such as sputtering. In addition, lead injection of electrodes, which is sometimes used in manufacturing multilayer components, is carried out in a reducing atmosphere at elevated temperatures. Such processing conditions may cause unstable electrical properties when the varistor is under a voltage stress. Such unstable electrical properties lead to a decreased varistor life.
It is believed that applicant is the first to successfully apply terminations with a nickel barrier to varistors by a vacuum deposition method known in the industry as sputtering. Sputtering avoids the problems and costs inherent in either a paste or a plating operation.
Sputtering is advantageous in that it is possible to deposit extremely thin layers of metallic material with the assurance that all surfaces subjected to the deposition procedure will be intimately engaged by the deposited metal. Thus, only the desired portion of the varistor will be contacted by the termination material. Thus, sputtering achieves favorable results over the plating of varistors with less problems and at a much lower cost.
However, a difficulty inherent in sputtering the termination materials still resides in that the deposited increments of metal will be received by all exposed portions of the varistor. Thus, unless the side faces of the varistor, that is, the faces between the ends to which terminations are to be applied are completely shielded from the sputtering operation, there is substantial likelihood of forming a film of sputtered material extending between the ends of the varistor, thereby short-circuiting the varistor. In addition, as noted above, the varistor material may suffer degradation of electrical properties when subjected to a reducing atmosphere at elevated temperatures, such as those encountered during sputtering.
In order to render sputtering commercially feasible as a means of terminating varistors, it is important that hundreds or even thousands of varistors be simultaneously treated. While conceptually sputtering could be simultaneously applied to a plurality of varistors imbedded in a plastic block or the like, the difficulties in aligning the varistors, casting the block, removing the surface portions of the block to expose the terminal ends of the varistors and dissolving the block after sputter applications, renders the method commercially impractical.
The applicant has used several techniques for sputtering terminations on varistors. One technique for effecting sputtered termination of varistors is the "close-pack method." In this technique, sputter termination is applied by fitting a plurality of varistors into a specially formed metallic jig or die which so closely embraces the sides of the varistors as to preclude the formation of a film of sputtered material on the side faces of the varistors during metal deposition. In effect, the surrounding ceramic bodies adjacent to a particular body provide the "mask" for the side faces of that body. Thus, this method requires that the fabrication of the bodies and the loading of the die be of precise dimensions capable of handling large quantities of varistors in a single run.
Another technique is to sputter the terminations on the ceramic bodies while shielding the portions of the varistors which are to remain free of sputtered material by implanting the varistors in an elastomeric block or slab having apertures sized to intimately engage side portions of the varistors while exposing their ends. This technique is described in detail as being applicable to capacitors in U.S. Pat. No. 4.,561,954 to Scrantom et al. ("Scrantom") which is incorporated herein by reference.
When the length of the mask is slightly less than the length of the capacitor, Scrantom permits the manufacture of "lands," terminated end portions which cover not only the ends of the capacitors but extend slightly along the side margins of the capacitors.
The technique of Scrantom is useful for applying terminations to the ends of varistors, but limits the number of varistors that can be terminated at one time since the elastomeric mask occupies a significant portion of the area where additional varistors could be located in the close-pack method.
Additionally, while such elastomeric material form an adequate shield, the material tends to "out-gas" in the course of the sputtering operation which is necessarily carried out under vacuum conditions. The result of such "out-gasing" is the formation at the interface between the deposited sputtered material and the varistors, of foreign increments or inclusions. The increments or inclusions result in the sputtered material making poor electrical contact with the electrodes and having poor adhesion with the ceramic. However, prior application to the mask of a sputtering layer or layers can avoid the out-gasing problem while leaving the mask sufficiently deformable to permit the varistors to be bodily shifted from a load plate into complemental positioned apertures formed in the plate.
There is thus a need to develop a commercially practical method which combines the advantage of the close-pack method for high-density loading and of the elastomeric block method for the ability to provide "lands." It would be desirable if the technique also could be used not only to apply terminations to varistors, but terminations to other electrical components such as capacitors and resistors. Additionally, it would desirable if the technique permitted the manufacture of "lands" like the elastomeric method described in Scrantom without "robbing" useful space in the die where additional varistors could be placed. This simply cannot be achieved in the close-pack method.